This is a very controversial question on the Olympus SLR Forum, they
argue that 4/3 lenses were designed with 4/3 sensor in mind from the
very beginning. Not like full frame lenses used on APS-C sensor of
other brands.

Interesting that this debate is going on there as well.

Ron Parr
wrote:

In reality, we almost never know about or care about the exact size of the
image circle projected by our lenses. We usually just know that it's large
enough and it's purely a supposition based upon an assumption of
economy that the image circle is no larger than required the typical use
of the lens.

You've stated my argument most eloquently!

I think the reason we have a disagreement here is because some folks have a fairly narrow, technical notion of what the term "crop factor" should mean, whereas I'm a pragmatist and accept that the term is actually in much wider use. Kind of like when generic ASA is refered to as "Aspirin". The Bayer company gets upset, but there's not too much they can do about it because the word has entered the vernacular.

It starts with the fact that sensors smaller than 35mm film are called "cropped sensors", and is reinforced by the fact that everyone describes field of view by referring to "35mm equivalent" focal lengths, thereby creating the need for a conversion factor. The conversion factor, which is technically called the "Focal Length Multiplier", is the same number calculated the same way as the "crop factor" on small-sensor DSLRs. And since "Crop Factor" rolls off the tongue more easily then "Focal Length Multiplier" and logically associates with the notion of a "cropped sensor", it's become the defacto term for the conversion factor.